Many domestic animals such as fowl and chicken usually have grit and feathers in their dung, for example, chicken have short digestive tract but no teeth, food is grinded by means of muscular stomach having grit so that food can be grinded to help digestion, thus a great deal of grit is contained in chicken dung and chicken feathers are mixed in the chicken dung; it will cause problems when using the chicken dung as the material of fermentation, pretreatment of separating the grit in the dung needs to be carried out before entering an anaerobic treatment tank so as to prevent the problems of the grit depositing in the anaerobic treatment tank, and the blocking of pipeline and pump and low gas yield in operation process, and the like; therefore, how to remove the grit in the chicken dung and the dung of other domestic animals is very important. The method of natural deposition and hand salvaging is used in the conventional chicken dung degriting; since the chicken dung contains a great deal of grit with tiny grain size and has high viscosity, the grit cannot be completely removed through the above method; moreover, the method wastes time and labor and is relatively difficult to carry out.